Ring transmission system and squelch method used for same

ABSTRACT

A ring transmission system of a bi-directional line switched ring (BLSR) type comprising nodes (A) to (F) connected by ring transmission lines RL, wherein the nodes (A) to (F) have modified squelch tables [A] to [F] and squelch decision units. The ring topology is built by transmitting a ring topology frame and inserting the ID of each node in that frame. Each of the nodes creates a modified squelch table comprised of modified node IDs given to the nodes in a rising order of connection starting from itself using itself as &#34;0&#34; or another reference value. Whether or not to perform a squelch operation is determined by comparing the magnitude of the modified node IDs of the nodes which signals cannot reach at the time of occurrence of a failure and the modified node IDs in the modified squelch table. This enables the squelch processing to be performed at a high speed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a ring transmission system of thebi-directional line switched ring type and to a squelch method of thesystem.

Systems of a plurality of nodes connected by ring transmission lines canbe roughly classified into the uni-directional path switched ring (UPSR)type and the bi-directional line switched ring (BLSR) type.

Compared with the former uni-directional path switched ring type, thelatter bi-directional line switched ring type enables use of the samechannel among different nodes and thus has the advantage that the linecapacity can be made larger. In this bi-directional line switched ringtype, when a failure occurs at a number of positions and the ringtransmission line is disconnected, a signal which cannot reach itsdestination node is generated. That signal is sometimes transmitted toanother node by the loopback function used so as to repair the failure.In this case, erroneous inter-node communication would occur, thereforea so-called "squelch" operation in which the signal which cannot reachits destination node is replaced by a path alarm indication signal(P-AIS) which is transmitted instead is carried out. There is a demandfor this squelch processing to be carried out with a good efficiency andat a high speed.

2. Description of the Related Art

As will be explained in detail later by using the drawings, a squelchtable is formed at each node in correspondence with the channels usingthe ID numbers of the transmission nodes (nodes to which the signals areadded) and the ID numbers of the reception nodes (nodes on which thesignals are dropped) at the time of setting up the communicationchannels among the nodes. When all of the control for insertion of asquelch operation using this squelch table when a number of failuresoccur is carried out by software, the processing for searching throughthe squelch table based on the ID numbers for the nodes not able toreceive signals is carried out in correspondence with the channels by aprocessor. In this case, the collation and comparison of the ID numbersare carried out one after the other, therefore there is the problem thata long time is required for a squelch decision.

Further, in a bi-directional line switched ring (BLSR) type ringtransmission system comprised of optical fiber transmission lines of alength of 1200 km and 16 nodes, it is desired that the time from thedetection of a failure to the loopback for repairing the failure and thecompletion of the switch between the working and protection lines be notmore than 50 ms. Further, when a plurality of failures occur, it hasbeen desired that the time be not more than 100 ms. Accordingly, itbecomes necessary to carry out the switching at a high speed after thedetection of a failure. Further, in the case of a plurality of failures,squelch processing becomes necessary, so this squelch processing mustalso be carried out at a higher speed.

Therefore, consideration may be given to a configuration which enablesall of the squelch processing to be all carried out by hardware.However, since a squelch table stores the node ID numbers incorrespondence with the channels and in correspondence with thetransmission or reception direction of the signals, there is a problemthat the circuit configuration for collating and comparing the squelchtable and the ID numbers of the nodes which signals cannot reach due toa plurality of failures becomes very complex and large in scale, thusrealization is difficult.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to achieve an increaseof the speed of the squelch processing without an increase of the sizeof the circuit.

To attain the above object, the present invention provides a ringtransmission system, for example, of a bi-directional line switched ring(BLSR) type comprising nodes (A) to (F) connected by ring transmissionlines RL, wherein the nodes (A) to (F) have modified squelch tables [A]to [F] and squelch decision units. The ring topology is built bytransmitting a ring topology frame and inserting the ID of each node inthat frame. Each of the nodes creates a modified squelch table comprisedof modified node IDs given to the nodes in a rising order of connectionstarting from itself using itself as "0" or another reference value.Whether or not to perform a squelch operation is determined by comparingthe magnitude of the modified node IDs of the nodes which signals cannotreach at the time of occurrence of a failure and the modified node IDsin the modified squelch table. This enables the squelch processing to beperformed at a high speed.

BRIEF DESCRIPTION OF THE DRAWINGS

The above object and features of the present invention will be moreapparent from the following description of the preferred embodimentswith reference to the accompanying drawings, wherein:

FIGS. 1A, 1B, and 1C are explanatory views of building the ring topologyin an embodiment of the present invention;

FIGS. 2A and 2B are explanatory views of a method of forming a squelchtable (first);

FIGS. 3A and 3B are explanatory views of a method of forming a squelchtable (second);

FIG. 4 is an explanatory view of a modified squelch table of anembodiment of the present invention;

FIG. 5 is an explanatory view of the unit configuration of a node;

FIG. 6 is an explanatory view of a switch operation in a node when afailure occurs;

FIG. 7 is a functional block diagram of the control of a node;

FIGS. 8A, 8B, 8C, 8D, 8E, and 8F are explanatory views of a squelchdecision (first);

FIGS. 9A, 9B, 9C, 9D, 9E, and 9F are explanatory views of a squelchdecision (second);

FIG. 10 is an explanatory view of squelch processing;

FIG. 11 is an explanatory view of an east side squelch decision unit(first);

FIG. 12 is an explanatory view of an east side squelch decision unit(second);

FIG. 13 is an explanatory view of a west side squelch decision unit(first);

FIG. 14 is an explanatory view of a west side squelch decision unit(second);

FIG. 15 is an explanatory view of a squelch insertion point;

FIG. 16 is an explanatory view of a signal flow when loopback is formedat the east side;

FIG. 17 is an explanatory view of a signal flow when loopback is formedat the west side;

FIGS. 18A and 18B are explanatory views of the squelch processing forthe protection line channels;

FIGS. 19A and 19B are explanatory views of the squelch processing at aswitching node;

FIG. 20 is an explanatory view of the squelch processing for theprotection line channels;

FIG. 21 is an explanatory view of the squelch processing for theprotection line channels;

FIG. 22 is an explanatory view of the time when a service selector isactivated;

FIG. 23 is an explanatory view of a squelch table when a serviceselector is activated (first);

FIG. 24 is an explanatory view of the squelch table when a serviceselector is activated (second);

FIG. 25 is an explanatory view of path setting states among nodes;

FIG. 26 is an explanatory view of a squelch table (first);

FIG. 27 is an explanatory view of a squelch table (second);

FIG. 28 is an explanatory view of ring transmission lines at the time ofoccurrence of a plurality of failures;

FIG. 29 is an explanatory view of a modified squelch table (first);

FIG. 30 is an explanatory view of a modified squelch table (second);

FIGS. 31A, 31B, 31C, and 31D are explanatory views of repair of afailure;

FIG. 32 is an explanatory view of an automatic protection switch (APS)protocol;

FIG. 33 is an explanatory view of a header and K1 and K2 bytes;

FIG. 34 is an explanatory view of a squelch operation;

FIGS. 35A and 35B are explanatory views of discrimination between asingle failure and a plurality of failures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before describing the embodiments of the present invention, the relatedart and the disadvantages therein will be described in further detailwith reference to the related figures.

FIGS. 31A, 31B, 31C, and 31D are explanatory views of repair of afailure, in which a uni-directional path switched ring (UPSR) typetransmits the same signal from a node (C) to an EW (East→West) side ofthe node (B) direction and to a WE (West→East) side of the node (D)direction by for example a channel ch.1 as shown in FIG. 31A, and thenode (A) selectively receives the signal of the channel ch.1 by a pathswitch PathSW. Accordingly, for example, even if a failure occursbetween the nodes (A) and (B) as shown in FIG. 31B, the node (A) canselectively receive the signal of the channel ch.1 via the node (D) bythe path switch PathSW, therefore the communication between the nodes(C) and (A) can be still maintained.

In the bi-directional line switched ring (BLSR) type, as shown in FIG.31C, the node (C) transmits the signal to the node (A) by for examplethe EW side channel ch.1 and transmits the signal to the node (D) by thechannel ch.1 on the WE side, while the node (D) can transmit the signalto the node (A) by the channel ch.1 on the WE side. Namely, by using thesame channel ch.1, communication, for example, between the nodes (C) and(A), between nodes (C) and (D), and between the nodes (D) and (A) becomepossible, so the line capacity can be made larger in comparison with theuni-directional line switched ring (UPSR) type.

In this bi-directional line switched ring (BLSR) type, as shown in FIG.31D, if a failure occurs between the nodes (A) and (B), it is repairedby the automatic protection switch (APS) protocol. At the node (B), forexample, the channel ch.1 is looped back to the channel ch.25 of theprotection line indicated by the thin line. At the node (A), the channelch.25 of the protection line is switched to the channel ch.1. Byswitching the signal from the node (C) transmitted on the channel ch.1to the channel ch.25 of the protection line at the node (B) and loopingit back and switching the channel ch.25 of the protection line to thechannel ch.1 of the working line at the node (A), the communicationbetween the channels (C) and (A) can be still maintained. Note that, thesignal between the nodes (C) and (D) and the signal between the nodes(D) and (A) do not pass between the nodes (A) and (B), therefore thecommunication is carried out by the channel ch.1.

FIG. 32 is an explanatory view of the automatic protection switch (APS)protocol, in which WK denote working lines, and PT denote protectionlines. Assume that a failure occurs between the nodes (A) and (B). Inthis case, the node (A) detecting an alarm becomes the switching nodeand transmits a request indicating the transmission line failure(SF-RING: Signal Failure Ring) to both of the short path and long pathwith respect to the opposing node (B). The nodes (D) and (C) receivingthe request via the long path identify the destination (B) of therequest and when recognizing that they are not the destination enterinto a "full pass through" state and allow the K1 and K2 bytes andprotection line channels to pass therethrough.

Further, the node (B) receiving the request via the short path becomesthe switching node and transmits a reverse request (RR-RING: ReverseRequest Ring) to the short path and a request the same as the receivedrequest (SF-RING) to the long path. In the case of a failure in thetransmission line, when receiving a request via the long path, a bridgeand switch are simultaneously formed. The bridge represents a statewhere the same traffic is sent out to the working and protectionchannels, while a switch represents a state where the traffic from theprotection channel is selected.

Accordingly, due to the occurrence of a failure between the nodes (B)and (A), in the node (A), a bridge for transmitting the signal to thenode (C) over the protection line PT is formed. This protection line PTforms at the node (B) a switch for transmitting the signal over theworking line WK from the node (B) toward the node (C). Further, in thenode (B), a bridge for looping back to the protection line PT the signalfrom the node (C) to the node (A) via the working line WK is formed. Inthe node (A), a switch for switching from this protection line PT to theworking line is formed. Accordingly, the communication between the nodes(A) and (C) is still maintained.

FIG. 33 is an explanatory view of the header and the K1 and K2 bytes andshows a frame configuration of a synchronous transport module (STM)-0(52 Mbps) consisting of 9 rows×90 bytes and a virtual container VC-3consisting of 9 rows×85 bytes. The section overhead SOH consisting of 9rows×3 bytes in the STM-0 frame includes the bytes of the framing bytesA1 and A2, STM-1 ID number C1, error monitor B1, orderwire E1, failureidentification F1, data communications D1 to D3, pointers (AU PRT) H1 toH3, error monitor B2, APS use K1 and K2, data communications D4 to D12,standby bytes Z1 and Z2 and an orderwire E2.

Further, the path overhead POH includes the bytes of a path connectionmonitor J1, path error monitor B3, path information ID C2, errornotification G1, maintenance channel F2, indicator H4 of the multi-framenumber, and standby bytes Z3 to Z5. VC-3 is multiplexed on the payloadof the 9 rows×87 bytes of the STM-0. Further, the STM-1 (156 Mbps) iscomprised by multiplexing the STM-0 in triplicate.

Further, the K1 byte in the section overhead SOH is comprised by arequest of first to fourth bits and an opposing office ID of fifth toeighth bits (ID number of the destination node of K1 byte), while the K2byte is comprised by a home office ID (ID number of the requestoriginating node) of the first to fourth bits, a fifth bit indicatingwhether the request is a short path request ("0") or a long path request("1"), and a status of the sixth to eighth bits. A request in the K1byte represents the SF-RING mentioned before by "1011", represents theRR-RING mentioned before by "0001", and represents "no request" by"0000". Further, the status in the K2 byte represents an alarmindication signal (AIS) by "111". The same is true also in the STS-1signal of SONET (52 Mbps).

FIG. 34 is an explanatory view of the squelch operation. As mentionedbefore, when the communication is carried out by using the channel ch.1between the nodes (A) and (C), between the nodes (A) and (D), andbetween the nodes (C) and (D), the squelch tables corresponding to thechannel ch.1 of each node store the IDs of the transmission nodes (nodesto which the signals are added) and the reception nodes (nodes on whichthe signals are dropped) for every transmission direction of thesignals. For example the squelch table in the node (C) stores the ID ofthe reception node (A) and the ID of the transmission node (C) to betransmitted to the node (B) side and the ID of the reception node (D)and the ID of the transmission node (C) to be transmitted to the node(D) side. Namely, the node ID numbers are stored in terms ofarrangements of the transmission nodes and reception nodes according tothe signal transmission direction.

When failures occur between the nodes (A) and (B) and between the nodes(A) and (D), the node (A) will be isolated. When connection is made atthe node (B) assuming that the channel ch.1 of the working line islooped back to the channel ch.25 of the protection line and at the node(D) assuming that the channel ch.1 between the node (D) and the node (A)is looped back to the channel ch.25 of the protection line, erroneousconnection occurs where the signal of the node (A) is transmitted to thenode (D).

Therefore, it can be found that, when looking up the squelch tables ofthe nodes (B) and (C) which are working as the switching nodes, thereare signals destinated to the node (A), even though the node (A) is nowunable to receive such signals. Therefore, in the node (B), a squelch(path alarm indication signal: P-AIS) indicated by S in the figure isinserted into the channel ch.25 of the protection line to be looped backto the node (A). In the node (D), a squelch (path alarm indicationsignal: P-AIS) indicated by S is inserted into the channel ch.1 of theworking line with the node (A) switched from the protection line. Theerroneous connection state can be avoided by this squelch.

FIGS. 35A and 35B are explanatory views of a decision of whether thefailure is a single failure or a plurality of failures. In FIG. 35A,when a failure occurs between the nodes (F) and (E) and the node (E)detects the transmission line failure SF (Signal Fail), it transmits tothe opposing node (F) a request (transmission line failure) indicated bySF-R/F/E/Long via the long path to the node (D) side and a request(transmission line failure) indicated by SF-R/F/E/Srt/RDI via the shortpath to the node (F) side. The node (F) can receive requests addressedto itself through the short path and long path from the opposing node(E) and therefore it can decide whether there is a single failurebetween the nodes (F) and (E).

Further, FIG. 35B shows a case where failures occur between the nodes(B) and (C) and between the nodes (F) and (E), the node (C) detects thetransmission line failure SF, and the node (E) detects the transmissionline failure SF. The node (C) transmits the requests (transmission linefailure) SF-R/B/C/Long and SF-R/B/C/Srt/RDI through the long path andthe short path to the opposing node (B). The node (B) receiving therequest SF-R/B/C/Srt/RDI via the short path transmits the requestSF-R/C/B/Long via the long path to the node (C).

In the same way as the case of FIG. 35A, the node (E) detecting thetransmission line failure SF transmits the requests SF-R/F/E/Long andSF-R/F/E/Srt/RDI through the long path and the short path to theopposing node (F). The node (F) transmits the request SF-R/E/F/Long viathe long path to the node (E) by the request SF-R/F/E/Srt/RDI via theshort path.

Accordingly, the node (F) can determine that a plurality of failureshave occurred between the nodes (F) and (E) and between the nodes (B)and (C) since the request SF-R/F/E/Srt/RDI via the short path wasaddressed to it from the node (E), while the request SF-R/C/B/Long viathe long path was not addressed to it, but was addressed to another node(C) from the node (B).

The node status changes among an idle state, a switching state, and apass through state. In the normal case, it is in the idle state. When afailure occurs, each node changes to either of a switching state formingeither or both of a bridge for switching the signal of the working linechannel to the protection line channel and a switch for returning fromthe protection line channel to the working line channel or a passthrough state where it is located between nodes of this switching state.

Further, the automatic protection switch (APS) protocol is executedbetween the nodes straddling the point where the failure occurred at theside where the failure occurred (short path) and the opposing side (longpath). The nodes at intermediate positions thereof enter into the passthrough state and monitor the automatic protection switch (APS)protocol, but do not terminate the K bytes of the automatic protectionswitch (APS) code. Then, a squelch operation is carried out to insert aP-AIS to the channels which are no longer in the normal connection statedue to the occurrence of a plurality of failures.

According to the conventional automatic protection switch (APS) protocolmentioned above, however, there are several problems, as alreadymentioned, that is:

(i) A squelch table is formed at each node in correspondence with thechannels using the ID numbers of the transmission nodes (nodes to whichthe signals are added) and the ID numbers of the reception nodes (nodeson which the signals are dropped) at the time of setting up thecommunication channels among the nodes. When all of the control forinsertion of a squelch operation using this squelch table when a numberof failures occur is carried out by software, the processing forsearching through the squelch table based on the ID numbers for thenodes not able to receive signals is carried out in correspondence withthe channels by a processor. In this case, the collation and comparisonof the ID numbers are carried out one after the other, therefore thereis the problem that a long time is required for a squelch decision.

(ii) Further, in a bi-directional line switched ring (BLSR) type ringtransmission system comprised of optical fiber transmission lines of alength of 1200 km and 16 nodes, it is desired that the time from thedetection of a failure to the loopback for repairing the failure and thecompletion of the switch between the working and protection lines be notmore than 50 ms. Further, when a plurality of failures occur, it hasbeen desired that the time be not more than 100 ms. Accordingly, itbecomes necessary to carry out the switching at a high speed after thedetection of a failure. Further, in the case of a plurality of failures,squelch processing becomes necessary, so this squelch processing mustalso be carried out at a higher speed.

(iii) Therefore, consideration may be given to a configuration whichenables all of the squelch processing to be all carried out by hardware.However, since a squelch table stores the node ID numbers incorrespondence with the channels and in correspondence with thetransmission or reception direction of the signals, there is a problemthat the circuit configuration for collating and comparing the squelchtable and the ID numbers of the nodes which signals cannot reach due toa plurality of failures becomes very complex and large in scale, thusrealization is difficult.

The present invention provides a technique, explained in detail below,which enables achievement of an increase of speed of the squelchprocessing without an increase of the size of the circuit.

In a first aspect of the invention, there is provided a bi-directionalline switched ring (BLSR) type ring transmission system comprised of aplurality of nodes connected by ring transmission lines, wherein eachnode is provided with a modified squelch table establishing acorrespondence between modified node ID numbers showing the order,taking each node as a reference, of connection of nodes to the ringtransmission line and the regular ID numbers of the nodes, thus usingthe modified node ID numbers to show pairs of communicating nodes and isprovided with a squelch decision unit performing a squelch decision by acomparison of the magnitude between the modified node ID numbersindicating the pairs of communicating nodes stored in this modifiedsquelch table and the modified node ID numbers of the nodes whichsignals cannot reach due to occurrence of a failure.

In a second aspect of the invention, each node is provided with acontroller unit for managing a ring topology and the squelch table; aswitch handling unit carrying out an automatic protection switch (APS)protocol upon detection of a transmission line failure, detection ofdegradation of a signal quality, and detection of removal of the mainsignal handling units of the two sides and performing a switch request;and a main signal handling unit. This main signal handling unit has aline switch which performs a switch operation upon a switch request fromthe switch handling unit, a modified squelch table created based on thering topology and squelch table of the controller unit, a squelchdecision unit performing a squelch decision by referring to thismodified squelch table, and a squelch execution unit carrying out asquelch operation according to the result of the squelch decision.

In a third aspect of the present invention, there is provided a squelchmethod adopted to a bi-directional line switched ring (BLSR) type ringtransmission system comprised of a plurality of nodes connected by thering transmission lines, which includes a step where, when building thering topology necessary for forming the squelch table, the node issuingthe command for building the ring topology transmits a ring topologyframe in which its own ID number is added at the start, a step where thenext node receiving this frame inserts its own ID number in the order ofreceipt and then transfers the same, and a step where each node builds aring topology using its own ID number as the start based on the order ofthe node ID numbers inserted into the frame after passing around thering transmission line.

In a fourth aspect of the present invention, there are further provideda step where each node assigns modified node ID numbers to other nodesin a rising or descending order from itself as a reference based on thering topology and a step where each node creates a modified squelchtable having the modified node ID numbers obtained by translating thenode ID numbers of the squelch table.

In a fifth aspect of the present invention, each node assigns modifiednode ID numbers to the other nodes in a rising order making its ownmodified node ID number "0" and in accordance with the order ofconnection in a clockwise direction or a counterclockwise direction inthe ring transmission lines.

In a sixth aspect of the present invention, there are further provided astep of discriminating the nodes which signals cannot reach when afailure occurs between nodes and a step of performing a squelch decisionby a comparison of magnitude between the modified node ID numbers ofthese nodes and the modified node ID numbers of the modified squelchtable.

In a seventh aspect of the present invention, each node decides that asquelch operation is necessary when recognizing by referring to themodified squelch table the existence of a transmission node or areception node in a sub-ring different from the sub-ring which itbelongs to itself among the sub-rings formed by the occurrence of aplurality of failures.

In an eighth aspect of the present invention, the ring transmissionlines include working lines and protection lines and there are furtherprovided a step of setting the node status indicating either of an idlestate in a normal mode and both a pass through state and a switchingstate between the East side and the West side at the occurrence of afailure and a step of performing a squelch decision with respect to theprotection line channels in accordance with that node status.

In a ninth aspect of the present invention, there is further provided astep of performing a squelch operation before assigning time slots forall drop side protection line channels when the node status indicatesthe pass through state.

In a 10th aspect of the present invention, there is further provided astep of performing a squelch decision by referring to the modifiedsquelch table by the modified node ID numbers when the node statusindicates the switching state and, at the same time, carrying out asquelch operation before assigning time slots for all drop sideprotection line channels and carrying out a squelch operation for aninput portion and output portion of the nodes along the protection linechannels.

In an 11th aspect of the present invention, there is provided a squelchmethod adopted to a bi-directional line switched ring (BLSR) type ringtransmission system comprising a plurality of nodes connected by ringtransmission lines, which includes a step of compulsorily carrying out asquelch operation for the unused lines when deciding whether or not asquelch operation is necessary due to the occurrence of a failure.

In a 12th aspect of the present invention, each node stores its own IDnumber or its own modified node ID number for the unused line channelsin the squelch table or the modified squelch table.

In a 13th aspect of the present invention, there is further provided astep where each node notifies to the controller unit the results of itsdecisions, except the result of a compulsory squelch decision on theunused line channels, among the squelch decision results in the mainsignal handling unit.

FIGS. 1A to 1C are explanatory views of building a ring topology in anembodiment of the present invention. As shown in FIG. 1A, there isprovided a system comprised of four nodes (A) to (D) which are connectedby the ring transmission lines RLs. ID numbers are assigned to therespective nodes. For example, ID=15 is assigned to the node (A), ID=3is assigned to the node (B), ID=7 is assigned to the node (C), and ID=8is assigned to the node (D), respectively. Next, as shown in FIG. 1B,the node issuing the command for building the ring topology (ring map),for example, the node (A), <1> transmits in for example a clockwisedirection a ring topology frame in which "1" is written as the number ofthe nodes inserted and its own ID is inserted as first. The node (B) <2>rewrites the number of nodes inserted to "2" and inserts its own ID nextto the ID of the node (A) and transmits the result. The node (C) <3>rewrites the number of nodes inserted to "3" and inserts its own ID nextto the ID of the node (B) and transmits the result. The node (D) <4>rewrites the number of nodes inserted to "4" and inserts its own ID nextto ID of the node (C) and transmits the result.

The node (A) determines that the frame made a round trip since the firstinserted node ID is its own ID. As shown in FIG. 3C, it then adds an ENDflag to the end of the ring topology frame and transmits the result tonotify the other nodes of the completed ring topology frame <5> to <7>.Each node receiving this ring topology frame builds a ring topology inwhich it is set as the start. For example, in the node (A), the topologybecomes "15, 3, 7, 8", in the node (B), it becomes "3, 7, 8, 15", in thenode (C), it becomes "7, 8, 15, 3", and in the node (D), it becomes "8,15, 3, 7".

By building the ring topology explained above, it becomes easy for anode to transmit its own ID and the destination node ID with the K1 andK2 bytes by the automatic protection switch (APS) protocol. Further,each node creates a squelch table according to this ring topology.

FIGS. 2A and 2B and FIGS. 3A and 3B are views explaining for forming thesquelch table. Each of the nodes (A) to (D) has a squelch table where itstores the IDs of the nodes, but for simplification the IDs will beexplained by using the same references as those of nodes (A) to (D). Forexample, where the signal is transmitted and received via the nodes (B)and (A) between the nodes (C) and (D) as in <1> of FIG. 2A, the node (C)inserts its own ID "C" into the principal part of the illustrated tablein correspondence with the channels and transmits the same to the node(B) side to notify it that the end office is the node (C), while thenode (D) inserts its own ID "D" into the principal part of theillustrated table in correspondence with the channels and transmits thesame to the node (A) to notify it that the end office is the node (D).The asterisk marks and the star marks in this case indicate that thedestination is unknown.

Then, as shown in <2> of FIG. 2B, it is notified to the node (B) fromthe node (D) via the node (A) that the end office on the node (A) sideis the node (D), and it is notified to the node (A) from the node (C)via the node (B) that the end office on the node (B) side is the node(C).

Next, as shown in <3> of FIG. 3A, it is notified to the node (C) via thenode (B) that the end office is the node (D), and it is notified to thenode (D) via the node (A) that the end office is the node (C). By this,the home node ID "C" and the opposing node ID "D" are set in the squelchtable of the node (C), and the home node ID "D" and the opposing node ID"C" are set in the squelch table of the node (D).

Next, as shown in <4> of FIG. 3B, based on the completed squelch tablesof the nodes (C) and (D), it is notified from the node (C) to the node(B) that the unknown destination shown by the asterisk mark is the ID"D" of the node (D). Further, it is notified from the node (D) to thenode (A) that the unknown destination shown by the star mark is the ID"C" of the node (C). Further, as shown in <5>, it is notified from thenode (B) to the node (A) that the unknown destination shown by theasterisk mark is the ID "D" of the node (D), and it is notified from thenode (A) to the node (B) that the unknown destination shown by the starmark is the ID "C" of the node (C). By this, the nodes (A) and (B) alsocomplete squelch tables in correspondence with each channel between thenodes (C) and (D). Further, the same procedure can be followed forupdating the squelch tables. Note that at the occurrence of a failure,control is exercised so as to prohibit the updating of the squelchtables.

FIG. 4 is an explanatory view of a modified squelch table of anembodiment of the present invention. It shows the case of a systemcomprised of the nodes (A) to (F) connected via the ring transmissionlines RLs. The ID numbers of the nodes are assigned as 5, 11, 9, 7, 1,and 3, so the ring topology [D] in the node (D) becomes "7, 1, 3, 5, 11,9", and the ring topology [E] in the node (E) becomes "1, 3, 5, 11, 6,7".

The squelch table stores the IDs so that the left side becomes thetransmission node (source node to which the signal is added) and theright side becomes the reception node (destination node on which thesignal is dropped) for every transmission direction (DIREC) of thesignal for the East side and West side of each node where thetransmission direction thereof is E→W, and stores the IDs so that theright side becomes the transmission node and the left side becomes thereception node where the transmission direction is E←W. Note that, wherean optical fiber transmission line is comprised by the working linechannels ch.1 to ch.24 and the protection line channels ch.25 to ch.48,at the occurrence of a failure, the only channels saved by the loopbackare the channels ch.1 to ch.24 of the working lines. Therefore thesquelch table can also be created for the channels ch.1 to ch.24.

In the squelch table [E] of the node (E), as indicated at the leftbottom, it is seen that the transmission node in the case of the Eastside, that is, in the case of the transmission direction of E→W on thenode (F) side, is ID=3 and the reception node is ID=7 and that thetransmission node in the case of the West side, that is, in the case ofthe transmission direction of E→W on the node (D) side, is ID=3 and thereception node is ID=7. Note that, 3_(F) and 7_(D) in the figureindicate that the node IDs of the nodes (F) and (D) are 3 and 7,respectively.

Further, in the squelch table [D] of the node (D), as indicated at theleft top, it is seen that the transmission node in the case of the Eastside, that is, in the case of the transmission direction of E→W on thenode (E) side, is ID=3 and the reception node is ID=7, which is the homenode. Further, since the West side, that is, the node (C) side, does notperform the transmission and reception, the table stores the ID=7 of thehome node as IDs of the transmission node and reception node. Further,it similarly stores the IDs of the nodes also for the transmissiondirection of E←W.

For example, where failures occur between the nodes (A) and (F) andbetween the nodes (D) and (E), the node (E) learns that the signal willnot reach the nodes having the IDs of 5, 11, 9 and 7 from the West sideloopback caused by the failure in the transmission line between the node(E) and the node (D), the conditions of the node ID=1 indicating itselfand the ID=3 of the node (F) by the request via the long path from thenode (F), and the ring topology. Therefore, for the squelch decision, itcompares the IDs of the transmission node and reception node in thesquelch table [E]. In this case, ID=7 is the ID of a node which signalscannot reach, therefore the node performs a squelch operation.

As mentioned before, when a node which signals cannot reach is causeddue to a number of failures, it is necessary to collate the ID of thatnode and the IDs of the transmission nodes and reception nodes stored inthe squelch table. The need for performing a squelch operation isdecided by carrying out the collation corresponding to a plurality ofchannels, thus the amount of processing involved in the squelch decisionwas large. Therefore, in the present invention, a modified squelch tableis created.

The ring topology stores the IDs of the nodes in the order of connectionto the ring transmission lines by setting the ID of each home node atthe start. The nodes (D) and (E) therefore end up with the ringtopologies [D] and [E] of FIG. 4. There, one's own node is made "0" andsubsequent numbers are allocated successively in a rising order.Accordingly, although the ring topology [E] of the node (E) wasoriginally "1, 3, 5, 11, 9, 7" as mentioned before by the IDs of thenodes, the modified node IDs (Md ID) becomes "0, 1, 2, 3, 4, 5".Similarly, although the ring topology [D] of the node (D) was originally"7, 1, 3, 5, 11, 9" by the IDs of the nodes, the modified node IDs (MdID) becomes "0, 1, 2, 3, 4, 5".

The modified squelch table is created from the squelch table by usingthese modified node IDs. Accordingly, in the modified squelch table [D]of the node (D), as indicated at the top right, the transmission node inthe transmission direction of E→W on the East side becomes the modifiednode ID=2 (node ID=3), and the reception node in the same directionbecomes the modified node ID=0 (node ID=7, home node). Similarly, in themodified squelch table [E] of the node (E), as indicated at the bottomright, the transmission node in the transmission direction of E→W on theEast side becomes the modified node ID=1 (node ID=3), and the receptionnode in the same direction becomes the modified node ID=5 (node ID=7).

Then, as mentioned before, where failures occur between the nodes (D)and (F) and between the nodes (A) and (F), the node (E) learns that thenodes which signals cannot reach are the nodes having the node IDs of 5,11, 9, and 7 in the above case, but according to the modified node IDsof the present invention, it learns that the signals will not reach thenodes having the IDs of 2 to 5. Namely, it learns that signals will notreach the nodes of the modified node IDs equal to or greater than 2.Accordingly, it carries out the squelch operation with respect to thenodes of the modified node IDs equal to or greater than 2 in themodified squelch table [E]. This means that it is possible to perform asquelch decision by a simple comparison of the magnitude of the modifiednode IDs and that a squelch decision can be carried out at a high speedby a simple comparator.

FIG. 5 is an explanatory view of the units constituting each node, inwhich PW is a power source unit, SV is a supervising unit, MP is amicroprocessor unit, HS (high speed switch) is a switch handling unit,HR (high speed receiver) (1) and HR(2) are reception interface units, HM(high speed multiplexer) (1) and HM(2) are main signal handling units,and HT (high speed transmitter) (1) and HT(2) are transmission interfaceunits. For example, the reception interface unit HR(1) receives thesignal from the East side, and the reception interface unit HR(2)receives the signal from the West side, the transmission interface unitHT(1) transmits the signal to the West side, and the transmissioninterface unit HT(2) transmits the signal to the East side. Where thetransmission line is comprised by an optical fiber, it includes afunction of opto-electrical conversion and electro-optical conversion.Further, when the main signal handling unit HM(1) is defined as theworking type (WORK), the main signal handling unit HM(2) becomes theprotection type (PTCT).

FIG. 6 is an explanatory view of the switch operation of the nodes whena failure occurs and shows the switch operation by the receptioninterface units HR(1) and HR(2), the main signal handling units HM(1)and HM(2), and the transmission interface units HT(1) and HT(2), shownin FIG. 5. The main signal handling units HM(1) and HM(2) have the samestructure, in which 11 is a pointer processing unit, 12 is a ring switchunit, 13 is a switch squelch unit, 14 is a drop time slot assignmentunit (DROP TSA), 15 is an add time slot assignment unit (ADD TSA), 16 isa bridge squelch unit, and 17 is a ring bridge unit.

Further, the nodes are connected by two transmission lines. Eachtransmission line has 48 channels. The channels ch.1 to ch.24 amongthose 48 channels are defined as the working lines (WK) (working linechannels), and the channels ch.25 to ch.48 are defined as the protectionlines (PT) (protection line channels). Further, the pointer processingunit 11 discriminates the starting position of the signal multiplexed bythe pointer comprising the H1 to H3 bytes in for example the sectionoverhead SOH and controls the respective portions.

For example, when a failure occurs on the East side of the node and thereception interface unit HR(1) and the transmission interface unit HT(2)cannot be used as indicated by a mark x in FIG. 6, the ring switch unit12 in the main signal handling unit HM(1) switches the channels ch.24 toch.48 of the protection lines on the West side to the channels ch.1 toch.24 of the working lines on the East side, and the ring bridge unit 17bridges the channels ch.1 to ch.24 to the channels ch.25 to ch.48 on theWest side.

Accordingly, the signal addressed to the home node which is looped backby the channels ch.24 to ch.48 of the protection lines and received fromthe West side can be dropped in the drop time slot assignment unit 14.Further, the signal inserted into the channels ch.1 to ch.24 of theworking lines in the add time slot assignment unit 15 are transmittedthrough the channels ch.25 to ch.48 of the protection lines from theWest side.

Further, the channels ch.1 to ch.24 of the working lines at the Westside are bridged to the channels ch.25 to ch.48 of the protection linesby the ring bridge unit 17, and the channels ch.25 to ch.48 of theprotection lines at the West side are switched to the channels ch.1 toch.24 of the working lines in the ring switch unit 13. By this, aloopback returning the signal from the West side to the West side can beformed.

The switch squelch unit 13 and the ring squelch unit 16 carry out asquelch operation for inserting the P-AIS by referring to the modifiedsquelch table when a signal cannot reach the destination node due to theoccurrence of a number of failures as mentioned before.

FIG. 7 is a functional block diagram of the control of each node andindicates the supervising unit SV, the controller unit MP, the switchhandling unit HS, the main signal handling unit HM, the receptioninterface unit HR, and the transmission interface unit HT. Thecontroller unit MP builds the ring topology by the means as explainedreferring to FIGS. 1A to 1C and creates the squelch table by the meansas explained referring to FIGS. 2A to 2B and FIGS. 3A to 3B. Further,the main signal handling unit HM includes the line switch, squelchdecision unit, squelch execution unit, and the modified squelch table.The line switch carries out the switching according to the switchrequest of the switch handling unit HS.

The controller unit MP manages the ring topology and the squelch table,requests a default automatic protection switch (APS) protocol to theswitch handling unit HS when the ring topology has not been built, andtransfers the built ring topology to the switch handling unit HS.Further, the requests from the supervising unit SV such as forcompulsory switching, manual switching, and test switching are sent tothe switch handling unit HS.

The switch handling unit HS supervises the reception signal at thereception interface unit HR. When detecting a transmission line failureSF or signal degradation SD or when detecting two-side (WK and PT)failures HM-ALM1&2 where the main signal handling units HM of the twosides are removed, it executes an automatic protection switch (APS)protocol by using them as the switching trigger and transmits the switchrequest by the result of this to the main signal handling unit HM.Further, in the case of two-side failure of the main signal handlingunit HM, the status of the sixth to eighth bits in the K2 byte isnotified to the other node as "100".

The main signal handling unit HM refers to the modified squelch tablecreated based on the squelch table in the controller unit MP by theswitch request from the switch handling unit HS, while the squelchdecision unit decides whether or not the squelch operation is necessaryand executes a squelch operation for inserting the P-AIS into thepredetermined channel by the squelch execution unit. Further, the mainsignal handling unit HM performs the switching by the line switch andnotifies the result of squelch decision and the result of squelchexecution to the controller unit MP. Note that, at the occurrence of afailure, the updating of the squelch table is prohibited, accordinglythere is no guarantee of coincidence of the relationship between thecontent of the squelch table and the time slot assignment. Therefore,the squelch operation is also carried out with respect to the unusedlines (UNEQ). That squelch operation is not notified to the controllerunit MP. Further, the squelch operation is not carried out for a nodewhich is isolated by the failure of both of the East side and West side,in the case of a single failure, or when the home node is not aswitching node.

FIGS. 8A to 8F are explanatory views of the squelch decision. FIG. 8Ashows a system wherein six nodes (A) to (F) to which the nodeidentification numbers IDs similar to those of the system shown in FIG.4 are assigned are connected by ring transmission lines. When a signalis transmitted and received between the nodes (D) and (F), if failuresoccur between the nodes (A) and (F) and between the nodes (D) and (E),the node (D) forms a loopback on the East side upon detecting atransmission line failure SF. The ring topology [D] in this case is "7,1, 3, 5, 11, 9" as shown in FIG. 8B. The modified squelch table in thecase where the signal is transmitted and received with the node (F) isthe modified node ID=2 of the node (F) of the node ID=3. Therefore, asshown in FIG. 8C, the E→W direction and E←W direction on the East sidebecome "2, 0". Namely, this means that a path between 2[F] and 0[D] isformed.

By the execution of the automatic protection switch (APS) protocol inthe switch handling unit HS (refer to FIG. 7), the node which the signalcan reach, that is, the node (A) of the far end node ID=5 can bediscriminated, therefore the ID of this node (A) is translated to themodified node ID=3 and notified to the main signal handling unit HM.Namely, as shown in FIG. 8D, the modified far end node ID (FEID=3 [A])of the node (A) represented by "0011", the node status ST representingthe East side switch represented by "10", the bridge state Brrepresented by "1", and the switch state Sw represented by "1" arenotified. Note that, a node status ST of "00" means the idle state, of"01" means the pass through state, "10" means the East side switchingstate, and "11" means the West side switching state. Further, the farend node ID in the case of a single failure is made "000" indicating thehome node.

As mentioned before, the node (D) does not receive the signal from theEast side due to the East side failure. Namely, in the ring topology [D]shown in FIG. 8E, the signals from the nodes of the modified node IDs of1 or more do not reach the node (D). Further, the far end node (A) onthe West side is the node of the limit for receiving the signal from theWest side. Namely, in the ring topology [D] shown in FIG. 8F, thesignals from the nodes of the modified node IDs of 2 or less do notreach the node (D). Accordingly, in the ring topology [D] shown in FIG.8B, it is seen that the nodes which the signals cannot reach are thenodes of the modified node IDs of 1 and 2.

In this case, it is seen that the signal does not reach the nodes havingthe modified node IDs of 2 or less, therefore by referring to themodified squelch table, when there is a node receiving no signals, thatis, when there is a modified node ID of 2 or less, it can be decidedthat the squelch operation is necessary. Therefore, in the modifiedsquelch table shown in FIG. 8C, there is a modified node ID of 2 orless, so a squelch operation will be executed with respect to thischannel.

FIGS. 9A to 9F are explanatory views of the squelch decision, and FIG.9A shows the same system as that of FIG. 8A. Further, the ring topology[E] in the node (E) becomes "1, 3, 5, 11, 9, 7" as shown in FIG. 9B.Communication between the modified node IDs of 1 and 5 is carried outbetween the nodes (F) and (D). Therefore, this means that a path between1[F] and 5[D] is formed. Accordingly, as shown in FIG. 9C, the modifiedsquelch table [E] becomes "1, 5" for the West side.

Further, by the execution of the automatic protection switch (APS)protocol due to the failures between the nodes (A) and (F) and betweenthe nodes (E) and (D), as shown in FIG. 9D, "0001" (FEID=1[F]) obtainedby translating the ID of the far end node (F) to the modified node ID,the node status ST of "11" indicating the West side switch state, thebridge state Br of "1", and the switch state Sw of "1" are notified fromthe switch handling unit HS to the main signal handling unit HM.

Further, since it is a West side failure, in the ring topology [E] shownin FIG. 9E, no signal is received from the West side (right side).Further, in the ring topology [E] shown in FIG. 9F, the far end node isthe node (F), and no signal reaches the nodes of the modified node IDsof the values exceeding this modified node ID of 1. Accordingly, asshown in the ring topology [E] of FIG. 9B, it is seen that the signaldoes not reach the nodes having the modified node IDs equal to 2 ormore, therefore if there are two or more modified node IDs in themodified squelch table [E], it is decided that a squelch operation isnecessary. In this case, since there is the node (D) of the modifiednode ID=5, the squelch operation will be carried out for that channel.

FIG. 10 is an explanatory view of the squelch processing and shows thesquelch processing in the same state as FIG. 8A, in which S denotes aswitch side squelch, B a bridge side squelch, and dotted lines denoteprotection lines. The nodes (A), (D), (E), and (F) detect thetransmission line failures SF and therefore become the switching nodes.

Further, the figure shows, for the node (D), the transmission directionDIREC of for example the channel ch.1 in the modified squelch table [D]thereof. The transmission node in the E→W direction on the East side isthe node (F) having the modified node ID=2, the reception node is thehome node (D), the transmission node of the E←W direction is the homenode (D), and the reception node is the node (F) having the modifiednode ID=2. Due to a failure on the East side, the switch Sw is formed inthe E→W direction, the bridge Br is formed in the E←W direction, thebridge Br is formed in the E→W direction on West side, and the switch Swis formed in the E←W direction.

For example, the node (D) refers to the modified squelch table [D] andcarries out a squelch operation since the signal does not reach the node(F) having the modified node ID=2 of the channel ch.1. Further, the node(E), similarly, refers to the modified squelch table and carries out asquelch operation for the channel ch.1 for the node (D) receiving nosignals.

FIG. 11 and FIG. 12 are explanatory views of the East side squelchdecision unit, in which FIG. 11 shows for example the case of the bridgeon the East side in the node (E), and FIG. 12 shows the case of theswitch on the East side, respectively. Reference numerals 21, 22, 41,and 42 are AND circuits (&), 23 and 43 are gate circuits outputting "1"when FEID is not equal to 0, and 24 and 44 are inverters. Theseconstitute a channel common part.

Further, 25, 26, 45, and 46 are comparators (COMP), 27, 28, 47, and 48are gate circuits outputting "1" when the modified node ID of thetransmission node or the reception node is not 0, 29 is a gate circuit(ALL0) outputting "1" when both of the modified node IDs of thetransmission node and the reception node are 0 (home nodes), 30, 31, 33,50, 51, and 53 are AND circuits (&), 32 and 52 are OR circuits (OR), and34 and 35 are registers. These constitute a channel corresponding part.Further, the channels ch.1 to ch.24 are defined as the working lines,and the channels ch.25 to ch.48 are defined as the protection lines.

Further, in the case of the far end node ID=4 based on the ring topology[E] by using the modified node ID, the control information from theswitch handling unit HS to the main signal handling unit HM becomes"01001011". Namely, the FEID (far end modified node ID number) becomes"0100", the node status ST indicating the East side switch becomes "10",the BR indicating the bridge state becomes "1", and the Sw indicatingthe switch state becomes "1".

Further, in the modified squelch table [E], as mentioned before,corresponding to the East side of the node (E), West side, and thetransmission directions DIREC (E→W, E←W), respectively, are stored thetransmission node SRC (source node) and the reception node DEST(destination node) represented by the switch state Sw, the bridge stateBr, and the modified node ID.

In the channel common part in FIG. 11, the far end node ID (FEID) isoutput as <1>, the "1" of the start in the node status ST "10" is inputto the AND circuit 22, and the "0" next to this is inverted to "1" bythe gate circuit 23 and input to the AND circuit 21. Further, the "1"representing the bridge state Br is input to the AND circuit 21. Theoutput <3> in this case becomes "1" and indicates the east bridgeEAST/Br. Further, the output of the gate circuit 23 becomes "1" sinceFEID is not equal to 0, and the output <2> of the AND circuit 21 becomes"1". Further, to the AND circuit 42 of the channel common part in FIG.12 is input the switch state Sw of "1". The outputs <1> to <3> of thiscase become the same as the outputs <1> to <3> from the channel commonpart of FIG. 11.

Further, in the channel corresponding part, the comparators 25 and 45compare the FEID and the modified node IDs (East SRC Br, East SRC Sw)representing the transmission nodes on East side, and the comparators 26and 46 compare the FEID and the modified node IDs (East DEST Br, EastDEST Sw) representing the reception nodes on the East side. Where B<A,the output is made "1". Namely, "1" is output in the case of a modifiednode ID smaller than the FEID.

Further, when the modified node ID representing the transmission nodeand the reception node is not 0, that is, when it does not indicate thehome node, the outputs of the gate circuits 27, 28, 47, and 48 become"1". If the outputs of the comparators 25, 26, 45, and 46 are "1", theoutputs of the AND circuits 30, 31, 50, and 51 become "1" and arerespectively input to the AND circuits 33 and 53 via the OR circuits 32and 52, while when the outputs <2> of the AND circuits 21 and 41 of thechannel common part are "1", the outputs of the AND circuits 33 and 53become "1" and are respectively set in the registers 34 and 54, toindicate the channel for which performance of a squelch operation hasbeen decided. Further, where both of the modified node IDs representingthe transmission node and the reception node are 0, the output of thegate circuit 29 becomes "1". In this case, an unused line is indicated,therefore the output of "1" of the AND circuit 33 is set in the register34 in this channel corresponding part and performance of a squelchoperation on the unused channel is decided.

FIG. 13 and FIG. 14 are explanatory views of the West side squelchdecision unit, in which FIG. 13 shows for example the case of the bridgeon the West side in the node (E), and FIG. 14 shows the case of theswitch on the West side thereof, 61, 62, 71, and 72 are AND circuits(&), and 63 and 73 are gate circuits outputting "1" when FEID is notequal to 0. These comprise the channel common part.

Further, 64 and 74 are registers indicating channels for whichperformance of the squelch operation has been determined. Further, 65,66, 75, and 76 are comparators (COMP), 67 and 77 are OR circuits (OR),68 and 78 are AND circuits (&), and 69 is a gate circuit (ALL0)outputting "1" when both of the modified node IDs representing thetransmission node and the reception node are 0. These comprise thechannel corresponding part.

Further, in the case of the far end node ID=4 according to the ringtopology [E] using the modified node IDs, the control information fromthe switch handling unit HS to the main signal handling unit HM becomes"01001111". Namely, the FEID (far end modified node ID number) becomes"0100", the node status ST indicating the switch at the West sidebecomes "11", the Br indicating the bridge state becomes "1", and the Swindicating the switch state becomes "1".

The output <1> from the channel common part in this case is FEID, andboth of outputs <2> and <3> become "1". Further, the comparators 65, 66,75, and 76 in the channel corresponding part make the output "1" whenthere is a transmission node or reception node having a modified node IDrepresenting a value larger than FEID conversely to the comparators ofthe channel corresponding parts of FIG. 11 and FIG. 12. Further, whenthe output <2> from the channel common part is "1", if the outputs fromthe comparators 65, 66, 75, and 76 are "1", the outputs from the ANDcircuits 68 and 78 become "1". They are set in the registers 64 and 74to indicate the channels for which performance of a squelch operationhas been determined. Further, the gate circuit 69 discriminates theunused lines on the bridge side and it is indicated to carry out asquelch operation.

FIG. 15 is an explanatory view of a squelch insertion point andindicates the principal parts of the time slot assignment unit of themain signal handling unit HM. In the figure, (WEST) denotes the Westside, (EAST) denotes the East side, 80 denotes a selector (E/WSEL), 81to 84 denote the West side or East side time slot assignment units(TSA), 85 and 86 denote insertion units (INS), 87 to 90 denote squelchunits (SQ_(Br), SQ_(Sw)), 91 and 92 denote AIS units, 93 and 94 denoteswitch units (S), and 95 and 96 denote bridge units (B). Further, thesolid lines denote the working lines (ch.1 to ch.24), and dotted linesdenote the protection lines (ch.25 to ch.48).

The signal dropped by the East side or West side time slot assignmentunits 82 and 83 via the selector 80 (E/W SEL) is transmitted from theselector 80, and the signal to be added is input to the East side andWest side time slot assignment units 81 and 84. Further, for example theWest side bridge of the node is formed by the bridge unit 96. The bridgesquelch in this case is executed by insertion of P-AIS at the squelchunit 87. Further, the East side switch is formed by the switch unit 93.The switch squelch in this case is executed by the insertion of theP-AIS at the squelch unit 88.

FIG. 16 is an explanatory view of the signal flow when a loopback isformed at East side. In the figure, the same references as those in FIG.15 indicate the same parts. The figure shows a case where a loopback isformed on the West side of the node due to the occurrence of a failureat the West side thereof. Namely, the channels ch.1 to ch.24 of theworking lines at the receiving East side are looped back to the channelsch.25 to ch.48 of the protection lines at the transmitting East side bya route of 93→88→85→87→96 when indicated by numerals by the control ofthe insertion unit 85 and the bridge unit 96. The insertion of the P-AISis performed for the channel for which performance of a squelchoperation has been decided at the squelch unit 87 by the squelchdecision.

Further, the channels ch.25 to ch.48 of the protection lines at thereceiving East side are looped back to the channels ch.1 to ch.24 of theworking lines at the transmitting East side by the route of 94→89→86→90when indicated by numerals by the control of the switch unit 94 and theinsertion unit 86. The insertion of the P-AIS is performed for thechannel for which performance of a squelch operation has been decided atthe squelch unit 89 by the squelch decision.

FIG. 17 is an explanatory view of the signal flow when a loopback isformed at West side. In the figure, the same references as those in FIG.15 indicate the same parts. The figure shows a case where a loopback isformed on the East side of the node due to the occurrence of a failureat the East side thereof. The control information comprised by the farend node ID (FEID), the node status ST="10" (East side switch), thebridge Br="1", and the switch Sw="1" are transferred from the switchhandling unit HS to the main signal handling unit HM in this case. Thechannels ch.1 to ch.24 of the working lines at the receiving West sideare looped back to the channels ch.25 to ch.48 of the protection linesat the transmitting West side by the route of 94→89→86→90→95 whenindicated by numerals by the control of the insertion unit 86 and thebridge unit 95. The insertion of the P-AIS is performed for the channelfor which performance of a squelch operation has been decided at thesquelch unit 90.

Further, the channels ch.25 to ch.48 of the protection lines at thereceiving West side are looped back to the channels ch.1 to ch.24 of theworking lines at the transmitting West side by the route of 93→88→85→87when indicated by numerals by the control of the switch unit 93 and theinsertion unit 85, and the insertion of the P-AIS is performed to thechannel determined to perform squelch operation at the squelch unit 88.

FIGS. 18A and 18B are explanatory views of the squelch processing withrespect to the protection line channels. As shown in FIG. 18A, the nodes(A) to (F) are connected via the working lines of the solid lines andthe protection lines of the dotted lines. Assume that a failure occursbetween the nodes (A) and (F) as shown in FIG. 18B when the transmissionand reception of the signal is carried out by the route of the solidarrow by the protection channel access (PCA) path between the nodes (F)and (D). The ring switch is generated in the nodes (A) and (F),respectively, and the protection lines will be used. At this time, theprotection channel access (PCA) path between the nodes (F) and (D)becomes unable to be used. Accordingly, the insertion positions ofsignals into the protection line channels are all placed in the throughstate, and the squelch operation S for inserting the P-AIS is executedat the position where the signals are dropped from the protection linechannels. Namely, at the pass through node, all of the protection linechannels are placed in the through state, and the signal add is stopped,while the insertion of the P-AIS is carried out to the signal dropchannel.

FIGS. 19A and 19B are explanatory views of the squelch processing in theswitching node. As shown in FIG. 19A, the nodes (A) to (F) are connectedby the ring transmission lines (working lines represented by the thinlines in the diagram and the protection lines represented by the dottedlines). In the nodes (B) and (D), in a state where the switch handlingunit HS is removed, when the communication is carried out by forming theprotection channel access (PCA) paths as indicated by the wide linesbetween the nodes (C) and (F), between the nodes (F) and (E), andbetween the nodes (E) and (D), it is assumed that a failure has occurredin the working line of the thin line going from the node (A) to the node(F) and in the protection channel access (PCA) path of the wide line asshown in FIG. 19B. In this case, the squelch operation is executed atthe point S. Namely, the nodes (F) and (E) place the protection linechannels into the through state and start the squelch operation forinserting the P-AIS into the channels dropped from the protection lines.

Further, the automatic protection switch (APS) code from the node (A)does not travel to the nodes (C) and (D) if the switch handling unit HSof the node (B) is removed, therefore the nodes (C) and (D) maintain thenormal mode state. In order to notify such nodes (C) and (D) that theprotection lines cannot be used, a squelch operation is performed forinserting the P-AIS into the channels of the protection lines incomingfrom the position of failure in the nodes (A) and (F) which have becomethe switching nodes. The channels of the protection lines outgoing tothe positions of failure are placed in the through state. Alternatively,a squelch operation is performed for inserting the P-AIS into thechannels of the protection lines as well outgoing to the position offailure.

FIG. 20 is an explanatory view of the squelch processing with respect tothe protection line channels. In the figure, the same references asthose in FIG. 15 indicate the same parts. The node status ST in thecontrol information from the switch handling unit HS to the main signalhandling unit HM is "01", which indicates the pass through state.Namely, the protection line channels ch.25 to ch.48 at the East side areconnected to the protection line channels ch.25 to ch.48 at the Westside by the insertion unit 85. Further, the protection line channelsch.25 to ch.48 at the West side are connected to the protection linechannels ch.25 to ch.48 at the East side by the insertion unit 86. Inthis case, the AIS is inserted, from the alarm indication signal units91 and 92 before performing the time slot assignment, into all channelsto be dropped from the protection line channels via the time slotassignment units 82 and 83 and the selector 80.

FIG. 21 is an explanatory view of the squelch processing with respect tothe protection line channels. In the figure, the same references asthose in FIG. 15 indicate the same parts. The node status ST in thecontrol information from the switch handling unit HS to the main signalhandling unit HM is "10", which indicates the state of the East switch(failure). Namely, the signals through the protection line channelsch.25 to ch.48 from the West side are transmitted through the workingline channels ch.1 to ch.24 to the West side via the switch unit 93, thesquelch unit 88, the insertion unit 85, and the squelch unit 87. Then,the squelch operation is executed by inserting the P-AIS into theprotection line channels ch.25 to ch.48 to the West side, from thesquelch unit 87.

Further, signals in the working line channels ch.1 to ch.24 from theWest side are transmitted via the protection line channels ch.25 toch.48 to the West side by the route of 94→89→86→90→95 when indicatedonly by numerals. Further, the AIS is inserted, from the alarmindication signal units 91 and 92 before performing the time slotassignment, into all channels to be dropped, via the time slotassignment units 82 and 83 and the selector 80, from the protection linechannels ch.25 to ch.48 at the East side and West side.

FIG. 22 is an explanatory view of the time when the service selector isactivated. Reference numeral 100 is a service selector (SS), and 101 isa drop and continue unit. Where one transmission system 102 to which thenodes (A) to (F) are connected through the ring transmission line andanother transmission system 103 similar to this are connected via forexample the nodes (D) and (E) and the signal is transmitted and receivedbetween the node (A) and the node (not illustrated) of the othertransmission system 103 by the channel ch.1, the signal from the node(A) is dropped by the drop and continue unit 101 of the node (E) and, atthe same time, transmitted to the node (D). Further, the normal signal,either the signal received from the other transmission system 103 at thenode (D) or the signal received from the other transmission system 103at the node (E) is selected by the service selector 100 of the node (E)and transmitted. Thus, the two transmission systems 102 and 103 areconnected by a duplexed transmission line.

FIG. 23 and FIG. 24 are explanatory views of the squelch tables when theservice selector is activated. They indicate the squelch tables [A] to[F] and ring topologies [A] to [F] corresponding to the nodes (A) to(F), respectively. Note that, 0 to 5 indicated in the lower portion ofthe ring topologies [A] to [F] indicate the above mentioned modifiednode IDs. For example, the ring topology [A] of the node (A) indicatesthe order of connection in the clockwise direction starting from thenode (A) according to the connection configuration shown in FIG. 22. Themodified node IDs are successively given in rising order starting fromthe node (A) as 0.

In the ring topologies [B] to [F] corresponding to the other nodes (B)to (F) as well, similarly the order of connection in the clockwisedirection is indicated starting from the home node. The modified nodeIDs are given successively in a rising order from the home node as 0.Further, the squelch tables [A] to [F] create the modified squelchtables (illustration omitted) based on the modified node IDs.

Further, the squelch table [A] of the node (A) indicates that, as shownin FIG. 23, the transmission node of the E→W direction of the channelch.1 at the West side is A, the reception node is D, the transmissionnode in the E←W direction is D, and the reception node is A. Further,the respective squelch tables [B] and [C] of the nodes (B) and (C)indicate the state (-) where the channel ch.1 is not used.

Further, the squelch table [F] in the node (F) shows the transmissionnode A and reception node D at East side in the E→W direction of thechannel ch.1 and the transmission node A and the reception node D at theWest side and, at the same time, shows the transmission node D andreception node A at the East side in the E←W direction and thetransmission node D and the reception node A at the West side. Further,for the channel ch.1 of the squelch table [E] in the node (E), the samecontent as that of the channel ch.1 of the squelch table [F] in the node(F) is stored. Further, for the channel ch.1 of the squelch table [D] inthe node (D), the transmission node A and the reception node D in theE→W direction on the East side and the transmission node D and thereception node A in the E←W direction are respectively stored.

In this case, the node (E) activates the service selector 100 wherebyone signal to the node (A) from the two paths is selected. The signalfrom the node (A) is transmitted to the two paths by the drop andcontinue unit 101. In this case, the IDs are stored in the squelch tableby defining the nodes in which the signals are finally dropped and addedas the reception nodes and the transmission nodes, respectively.

For example, where a failure occurs in the channel ch.1 between thenodes (E) and (D), the AIS is inserted between the nodes (E) and (D),but the communication by the channel ch.1 between the node (A) and theother transmission system 103 can be continued via the node (E),therefore a squelch operation is not executed. Further, in the case of amulti-cast, a squelch table in which the node furthest from thetransmission node is defined as the reception node will be created.Further, by this squelch table and the modified node ID, a modifiedsquelch table is created. Further, in this case, if there is no failurewith the node nearest the transmission node, the communication can becontinued at least with the node nearest the transmission node,therefore no squelch operation is carried out between the two.

FIG. 25 is an explanatory view of the path setting states among thenodes. It shows a case where paths are set up between the nodes (A) and(B) via the channels ch.1 to ch.8 at the East side of the node (A),between the nodes (A) and (C) via the channels ch.9 to ch.16, betweenthe nodes (A) and (D) via the channels ch.17 to ch.24, between the nodes(A) and (F) via the channels ch.1 to ch.8 at the West side of the node(A), between the nodes (A) and (E) via the channels ch.9 to ch.16,between the nodes (A) and (D) via the channels ch.17 to ch.24, betweenthe nodes (C) and (E) via the channels ch.9 to ch.16 at the East side ofthe node (C), and between the nodes (E) and (F) via the channels ch.1 toch.8 at the East side of the node (E), respectively.

FIG. 26 and FIG. 27 are explanatory views of the squelch table. Forexample, the ring topology (A) in the node (A) shown in FIG. 26indicates the order of connection in the clockwise direction startingfrom the node (A) and indicates the fact that five nodes (B, C, D, E andF) are connected with itself, as "5" on the right side of [A]. Further,the squelch table [A] indicates that, for the channel ch.1, there arethe transmission node B and the reception node A at the East side in theE→W direction (DIREC), there are the transmission node A and thereception node F at the West side, there are the transmission node A andthe reception node B at the East side in the E←W direction, and thereare the transmission node F and the reception node A at the West side.For the channel ch.9, it indicates that there are the transmission nodeC and the reception node A at the East side in the E→W direction, thereare the transmission node A and the reception node E at the West side,there are the transmission node A and the reception node C at the Eastside in the E←W direction, and there are the transmission node E and thereception node A at the West side. Further, for the channel ch.17, thetransmission node and the reception node become A and D in both of theE→W direction and the E←W direction.

Further, for example, in the squelch table [D] of the node (E) shown inFIG. 27, the transmission node and the reception node are set to E and Cfor the E→W direction and the E←W direction of the channel ch.9,respectively, and similarly the transmission node and the reception nodeare set to A and D for the E→W direction and the E←W direction of thechannel ch.17, respectively

FIG. 28 is an explanatory view of the ring transmission lines when aplurality of failures occur. It indicates a case where the failuresoccur between the nodes (A) and (F) and between the nodes (E) and (D).It indicates a state where the ring transmission lines are divided tothe first sub-ring composed by the nodes (A) to (D) by the loopback inthe nodes (A) and (D) and the second sub-ring composed by the nodes (E)and (F) by the loopback in the nodes (E) and (F). Further, S indicates apoint for performing the squelch operation.

FIG. 29 and FIG. 30 are explanatory views of modified squelch tables andindicate the modified squelch tables [A] to [F] created by using themodified node IDs based on the squelch tables [A] to [F] shown in FIG.26 and FIG. 27 mentioned before. For example, the "_(B) " of the "1_(B)" in the modified squelch table [A] is attached so as to facilitate theunderstanding that the modified node ID=1 corresponds to the node B ofthe squelch table [A] of FIG. 26. The same is true also for the othersuffixes.

Further, the ring topologies [A] to [F] represent the ring topologiesindicated by A to F by using the modified node IDs while defining eachhome node as 0. Further, it indicates also the control informationcomprised of the far end node ID (FEID) from the switch handling unit HSto the main signal handling unit HM at the time of occurrence of thefailure shown in FIG. 28, the node status ST, the bridge Br, and theswitch Sw.

For example, in the node (A), due to the failure at the West side, aloopback using the protection line channels ch.25 to ch.48 is created atthe West side. At this time, the node (A) discriminates that thetransmission node of the automatic protection switch (APS) code changesfrom the node (F) to the node (D). This node (D) has the modified nodeID=3. The control information comprised of the FEID (=4), ST (=West sideswitch), Br, and Sw respectively having the values of "0011", "10", "1",and "1" is transferred from the switch handling unit HS to the mainsignal handling unit HM.

Therefore, the main signal handling unit HM refers to the modifiedsquelch table [A] to search for whether or not a node having a modifiednode ID of 4 or more is included. In this case, the channels ch.1 andch.9 include nodes having modified node IDs of 4 or more, therefore itis decided that the squelch operation is necessary for these channelsch.1 and ch.9. Further, the channel ch.17 becomes the loopback channelusing the protection line channels ch.41 to ch.48, therefore it is seenthat the communication between the nodes (A) and (D) is continued bythis channel ch.17 and the squelch operation is unnecessary.

Further, in the node (E), due to the failure at the East side, aloopback is created on the West side. At this time, the transmissionnode of the automatic protection switch (APS) code changes from the node(A) to the node (F), therefore the node (E) can discriminate thatsignals are not received at the nodes having modified node IDs of 2 ormore. The control information comprised by the FEID (=1), ST (West sideswitch), Br, and Sw respectively having the values of "0001", "11", "1",and "1" is transferred from the switch handling unit HS to the mainsignal handling unit HM in this case.

Therefore, the main signal handling unit HM refers to the modifiedsquelch table [E] and searches for nodes having modified node IDs of 2or more. By this, it is found that nodes having modified node IDs of 2or more are included in the channels ch.9 and ch.17, therefore it isdecided that the squelch operation is necessary.

The other nodes can perform operations similar to that explained aboveto refer to the modified squelch tables and carry out a squelchdecision. Further, where the ring transmission lines are divided intothe second sub-ring to which for example the node (E) belongs and thefirst sub-ring to which the node (D) belongs due to the occurrence of aplurality of failures, for example the node (E) can refer to themodified squelch table [E] and decide that the squelch operation isnecessary when the transmission node or the reception node exists in thesub-ring to which the node (D) belongs. Further, the squelch decisioncan be carried out by the comparison of magnitude of the modified nodeID, therefore the squelch decision circuit can be constituted by arelatively simple logical circuit shown in FIG. 11 to FIG. 14. Further,the squelch decision can be executed at a high speed by hardware.

As explained above, the present invention has the advantage that eachnode constituting the ring transmission system includes a modifiedsquelch table and a squelch decision unit and, at an occurrence of afailure, the squelch decision can be carried out by the comparison ofthe magnitude of the modified node ID numbers. Therefore the squelchdecision units can be realized by relatively simple comparators and, atthe same time, since these are hardware, a high speed decision becomespossible. Further, by transmitting the ring topology frame and insertingthe ID number of each node to build the ring topology, creating asquelch table relating to the path setting among the nodes based on thattopology, and creating a modified squelch table comprised of the node IDnumbers translated to modified node ID numbers using the home node asthe reference value, the squelch decision mentioned before can becarried out by a comparison of the magnitude of the modified node IDnumbers. Further, by performing a squelch operation with respect to thedrop channels of the protection lines or the unused lines, there is theadvantage that the occurrence of erroneous connection can be reliablyprevented.

What is claimed is:
 1. A bi-directional line switched ring type ringtransmission system comprised of a plurality of nodes connected by ringtransmission lines, whereineach node is provided with a modified squelchtable establishing a correspondence between modified node ID numbersshowing the order, taking each node as a reference, of connection ofnodes to the ring transmission line and the regular ID numbers of thenodes, thus using the modified node ID numbers to show pairs ofcommunicating nodes and a squelch decision unit performing a squelchdecision by a comparison of the magnitude between the modified node IDnumbers indicating the pairs of communicating nodes stored in thismodified squelch table and the modified node ID numbers of the nodeswhich signals cannot reach due to occurrence of a failure.
 2. A ringtransmission system as set forth in claim 1, wherein each node isprovided witha controller unit for managing a ring topology and thesquelch table; a switch handling unit carrying out an automaticprotection switch protocol upon detection of a transmission linefailure, detection of degradation of a signal quality, and detection ofremoval of the main signal handling units of the two sides andperforming a switch request; and a main signal handling unit, the mainsignal handling unit having a line switch which performs a switchoperation upon a switch request from the switch handling unit, amodified squelch table created based on the ring topology and squelchtable of the controller unit, a squelch decision unit performing asquelch decision by referring to this modified squelch table, and asquelch execution unit carrying out a squelch operation according to theresult of the squelch decision.
 3. A squelch method adopted to abi-directional line switched ring type ring transmission systemcomprised of a plurality of nodes connected by ring transmission lines,comprising the steps:where, when building the ring topology necessaryfor forming a squelch table, the node issuing the command for buildingthe ring topology transmits a ring topology frame in which its own IDnumber is added at the start, where the next node receiving this frameinserts its own ID number in the order of receipt and then transfers thesame, where each node builds a ring topology using its own ID number asthe start based on the order of the node ID numbers inserted into theframe after passing around the ring transmission line; where each nodeassigns modified node ID numbers to other nodes in a rising ordescending order from itself as a reference based on the ring topology,and where each node creates a modified squelch table having the modifiednode ID numbers obtained by translating the node ID numbers of thesquelch table.
 4. A squelch method adopted to a ring transmission systemas set forth in claim 3, wherein each node assigns modified node IDnumbers to the other nodes in a rising order making its own modifiednode ID number "0" and in accordance with the order of connection in aclockwise direction or a counterclockwise direction in the ringtransmission lines.
 5. A squelch method adopted to a ring transmissionsystem as set forth in claim 3, further provided witha step ofdiscriminating the nodes which signals cannot reach when a failureoccurs between nodes and a step of performing a squelch decision by acomparison of magnitude between the modified node ID numbers of thesenodes and the modified node ID numbers of the modified squelch table. 6.A squelch method adopted to a ring transmission system as set forth inclaim 4, further provided witha step of discriminating the nodes whichsignals cannot reach when a failure occurs between nodes and a step ofperforming a squelch decision by a comparison of magnitude between themodified node ID numbers of these nodes and the modified node ID numbersof the modified squelch table.
 7. A squelch method adopted to a ringtransmission system as set forth in claim 3, wherein each node decidesthat a squelch operation is necessary when recognizing by referring tothe modified squelch table the existence of a transmission node or areception node in a sub-ring different from the sub-ring which itbelongs to itself among the sub-rings formed by the occurrence of aplurality of failures.
 8. A squelch method adopted to a ringtransmission system as set forth in claim 4, wherein each node decidesthat a squelch operation is necessary when recognizing by referring tothe modified squelch table the existence of a transmission node or areception node in a sub-ring different from the sub-ring which itbelongs to itself among the sub-rings formed by the occurrence of aplurality of failures.
 9. A squelch method adopted to a ringtransmission system as set forth in claim 5, wherein each node decidesthat a squelch operation is necessary when recognizing by referring tothe modified squelch table the existence of a transmission node or areception node in a sub-ring different from the sub-ring which itbelongs to itself among the sub-rings formed by the occurrence of aplurality of failures.
 10. A squelch method adopted to a ringtransmission system as set forth in claim 6, wherein each node decidesthat a squelch operation is necessary when recognizing by referring tothe modified squelch table the existence of a transmission node or areception node in a sub-ring different from the sub-ring which itbelongs to itself among the sub-rings formed by the occurrence of aplurality of failures.
 11. A squelch method adopted to a bi-directionalline switched ring type ring transmission system comprised of aplurality of nodes connected by ring transmission lines, where the ringtransmission lines include working lines and protection lines,comprising the steps of;setting the node status indicating either of anidle state in a normal mode and both a pass through state and aswitching state between the East side and the West side at theoccurrence of a failure; performing a squelch decision with respect tothe protection line channels in accordance with that node status;performing a squelch decision by referring to a modified squelch tableby a modified node ID numbers when the node status indicates theswitching state; and, at the same time, carrying out a squelch operationbefore assigning time slots for all drop side protection line channels;and carrying out a squelch operation for an input portion and outputportion of the nodes along the protection line channels.
 12. A squelchmethod adopted to a ring transmission system as set forth in claim 3,wherein each node stores its own ID number or its own modified node IDnumber for the unused line channels in the squelch table or the modifiedsquelch table.
 13. A squelch method adopted to a ring transmissionsystem as set forth in claim 4, wherein each node stores its own IDnumber or its own modified node ID number for the unused line channelsin the squelch table or the modified squelch table.
 14. A squelch methodadopted to a ring transmission system as set forth in claim 5, whereineach node stores its own ID number or its own modified node ID numberfor the unused line channels in the squelch table or the modifiedsquelch table.
 15. A squelch method adopted to a ring transmissionsystem as set forth in claim 6, wherein each node stores its own IDnumber or its own modified node ID number for the unused line channelsin the squelch table or the modified squelch table.
 16. A squelch methodadopted to a ring transmission system as set forth in claim 12, whereinthere is further provided a step where each node notifies to acontroller unit the results of its decisions, except the result of acompulsory squelch decision on the unused line channels, among thesquelch decision results in a main signal handling unit.
 17. A squelchmethod adopted to a ring transmission system as set forth in claim 13,wherein there is further provided a step where each node notifies to acontroller unit the results of its decisions, except the result of acompulsory squelch decision on the unused line channels, among thesquelch decision results in a main signal handling unit.
 18. A squelchmethod adopted to a ring transmission system as set forth in claim 14,wherein there is further provided a step where each node notifies to acontroller unit the results of its decisions, except the result of acompulsory squelch decision on the unused line channels, among thesquelch decision results in a main signal handling unit.
 19. A squelchmethod adopted to a ring transmission system as set forth in claim 15,wherein there is further provided a step where each node notifies to acontroller unit the results of its decisions, except the result of acompulsory squelch decision on the unused line channels, among thesquelch decision results in a main signal handling unit.